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<br />driven with diesel or gas engines and increase the thermal efficiency <br /> <br /> <br />by recovering 'heat from the engine exhaust gas. Gas turbines have been <br /> <br />.... <br /> <br />CO proposed for the compressor drive for plants of 2-8 mgd, with heat recovery <br /> <br />-l <br />~~ from the turbine exhaust gas. <br /> <br />The selected compressor and driver in turn determine the optimum <br /> <br />number of effects, temperature differential per effect, and compressor <br /> <br />suction temperature and volume. Thus, the vapor compression process <br /> <br />plant capacity is to a large degree fixed by the available drivers and <br /> <br />compressors. The plants can be operated slightly above or below design <br /> <br />capacity with auxiliary firing or with bypassing, but this will result <br /> <br />in higher water costs. <br /> <br />One of the important vapor compression process variables is the <br /> <br />temperature of the boiling brine in the last effect. The process has <br /> <br />o <br />been run with.brine temperatures as low as 100 F; however, the efficiency <br /> <br />is improved at higher temperatures as a result of higher heat transfer <br /> <br />coefficients and a lower volume of vapor that must be handled by the com- <br /> <br />pressor. For this reason the compressor-suction temperature is usually <br /> <br />in the range of 210 to 240oF. The vapor from the compressor is super <br /> <br />heated, and it is necessary to inject sufficient condensate to produce <br /> <br />a saturated v~por so that condensation can occur in the first VTE effect. <br /> <br />As in all distillation processes, the VC process has maximum allowable <br /> <br />concentration :limits both at the upper and lower temperature limits, <br /> <br />depending on the type of feed water and feed pretreatment. With a sea- <br /> <br />water feed and acid pretreatment, the concentration ratio is limited <br /> <br />. 0 <br />to 2 at 225 F in the VTE section of the referenced plant. <br /> <br />The temperature differential across each effect (outside tube <br />temperature minus inside temperature) can be as low as 40F in some units. <br />However, most units are designed with 6_SoF differentials to reduce the <br /> <br />required area and to offset the effects of fouling, boiling point rise, <br /> <br />43 <br /> <br />